Radiogenic isotopes are used in geochemistry in two principal ways. Historically they were first used to determine the age of rocks and minerals. More recently they have been used in petrogenetic studies to identify geological processes and sources. The former application is normally described as geochronology, the latter as isotope geology or isotope geochemistry. There are a number of excellent texts which deal with these two disciplines (Faure, 1986; Jager and Hunziker, 1977; DePaolo, 1988) and the reader is referred to these for more detailed treatments of the topics covered here.